1. Field of the Invention
The present invention generally relates to reading of information from an optical recording medium and more particularly to a compact and high-density optical information detection apparatus capable of reproducing information from a high-density optical recording medium in which information is recorded on both a land and a groove that define a track. More specifically, the present invention relates to an optical information detection apparatus in which cross-talk between the information read out from the land and read out from the groove is minimized and wherein the resolution at the time of detection of the recorded information is improved.
Optical disks are used extensively as the recording medium of various information including audio and visual data. In relation to the art of high-density rewritable recording of information, intensive efforts are being made particularly with regard to the development of rewritable optical disks such as a magneto-optical disk or a phase transition disk.
In order to increase the recording density of such optical disks, it is desired to decrease the wavelength of the optical beam used for information detection or to increase the numerical aperture of the objective lens such that the beam spot of the optical beam on the recording medium is reduced.
Further, there is a proposal to use MSR (magnetic super-resolution) technology. It should be noted that the MSR technology attempts to increase the recording density of a magneto-optical recording medium while using the optical beam of the same spot size, by suppressing the cross-talk between the tracks or between the recording marks aligned in the tangential direction of the track as much as possible. However, the MSR technology still includes various problems related to resolution which appear conspicuously when the track pitch is reduced, such as the decrease of tracking performance or the increase of the crosstalk. In the case of a rewritable optical disk such as a magneto-optical disk, the cross-erasing of information becomes also a serious problem.
Meanwhile, there is a proposal of so-called land-groove recording technology that increases the effective track recording density twice as compared with the conventional land recording technology or groove recording technology. In the conventional land recording technology or groove recording technology, the information is recorded only on the land or on the groove that defines a track, while the information is recorded both on the land and the groove in the land-groove recording technology.
In the land-groove recording technology, in which lands and grooves are separated three-dimensionally, the problem of cross-erasing of information is effectively suppressed as a result of the spatial separation of the lands and the grooves. Thus, the land-groove recording technology is thought an effective approach to increase the recording density of optical disks including rewritable optical disks. In order to reduce this promising technology into practice, however, it is necessary to devise a method of suppressing the cross-talk further.
2. Description of the Prior Art
Conventionally, there is a proposal to reduce the cross-talk as described in the Japanese patent application 9-16134, wherein this prior application achieves the desired suppressing of the cross-talk between the lands and the grooves by applying a phase compensation to the optical signals produced by the lands and produced by grooves of the optical recording medium independently. When the desired increase of the line recording density is to be achieved according to this prior application while using the same spot size for the optical beam, on the other hand, there is a need of a further process for compensating for the decrease of the reproduced signal output. It should be noted that such a decrease of the reproduced signal output is caused by the interference of the recording marks aligned on a track.
With regard to the improvement of resolution of the reproduced signal output for the recording marks aligned on a track, there is a proposal of optical super-resolution by Milster, T. D., et al., Japanese J. Appl. Phys. vol.32, 1993, pp.5397-5401, in which a shading band is provided in the optical path which is used for detecting the information from an optical disk. Thereby, the shading band functions as an optical equalizer.
Further, in view of the recent trend of technology that targets an integrated optical head carrying a hologram, it is desired that the high-density recording method is compatible with the construction of such integrated optical heads.
Furthermore, there is a proposal of optical information detection method as disclosed in the Japanese Laid-Open Patent Publication 9-128825, in which simultaneous detection of different information is achieved by dividing a reflected optical beam into several optical beams by using one or more optical beam splitters. It should be noted that the process of this prior art achieves the optical beam splitting with respect to the entirety of the optical beam, by disposing the optical beam splitter so as to intercept the entire optical beam that is reflected by the optical recording medium and traveling toward an optical detection system.
With regard to the process of the Japanese patent application 9-16134 noted before, it is confirmed that the MSR process is an effective approach for suppressing the cross-talk between the tracks and the interference between the recording marks aligned on a track. On the other hand, the process of the foregoing prior application has a drawback in that it requires at least two magneto-optical layers on the magneto-optical recording medium and that a high optical power has to be used for the optical beam used for reading information. Further, there is an additional drawback in that an exact control of the optical beam power is necessary such that the optical beam power falls within a narrow tolerance range.
In addition to the foregoing, the process of the Japanese patent application 9-16134 has a drawback in that, while the problem of the cross-talk between the tracks may be successfully reduced, the reproduced optical beam tends to have an ecliptic polarization state due to the admixing of polarization components having a mutual phase offset corresponding to twice the depth of the groove, into the reflected optical beam. It should be noted that such an admixing of the polarization component occurs as a result of the reflection of the optical beam at the land and the groove adjacent thereto. When this occurs, the output of the reproduced signal is deteriorated inevitably. In order to avoid this problem, it is necessary to provide an appropriate optical phase compensation process.
It is possible to achieve the desired increase of the track recording density and the linear recording density without using the MSR technology, by combining the optical super-resolution of the Milster et al., op. cit., which uses a shading band, with the optical phase compensation process applied separately to the optical beams reflected from the lands and reflected from the grooves. However, such a process requires a construction in which the optical shading band and the optical phase compensation device are provided for each of the optical beams reflected by the lands and the grooves, and the construction of the optical system becomes inevitably bulky and complex.
In the process of Japanese Laid-Open Patent Publication 9-128825, which divides the reflected optical beam into a plurality of optical beam elements, on the other hand, there has been a problem in that it is difficult to construct the optical information detection apparatus to have a compact size, due to the fact that the beam splitting is applied to the entirety of the reflected optical beam at several locations of the optical path of the reflected optical beam and that it is necessary to provide a detection optical system to each of the optical beams thus divided.